Generally, when semiconductor devices, liquid crystal display devices, magnetic thin-film heads and the like are manufactured in a photolithography process, a projection exposure apparatus (hereinafter referred to as an exposure apparatus) is employed. A projection exposure apparatus transfers a pattern formed on a photomask or a reticle (hereinafter referred to as a reticle) to a photosensitive substrate wafer or a glass plate (hereinafter referred to as a wafer). Recently, patterns of semiconductor devices or the like are increasingly downsized to micropatterns. To realize such downsizing, it is necessary for an exposure apparatus to improve alignment precision. Among various deterioration factors of alignment precision, particularly distortion in a pattern image needs to be reduced.
To reduce distortion, naturally it is necessary to reduce aberrations in a projection optical system. To achieve this, a projection optical system employed in a conventional exposure apparatus is designed to satisfy conditions such that an average of aberrations and distortions becomes small in the entire projection field of view. To bring aberrations and distortions within a designed allowable range, lens devices and optical members are processed with high precision, then aberrations are actually measured, and complicated cumbersome processes of assembly, adjustment, and inspections, e.g., adjustment of air space between lenses, a gradient of a lens, parallel eccentricity of a lens, and the like, are repeatedly performed for constructing a projection optical system.
Among various aberrations, symmetrical components of the distortions with respect to the optical axis or regular asymmetrical components are adjustable by the aforementioned adjusting methods. However, random components of distortions cannot be adjusted by the aforementioned adjusting methods. In view of this, for instance, Japanese Patent Application Laid-Open No. 8-203805 discloses a technique for reducing some of the difficulties involving manufacturing of such highly precise projection optical system, and for bringing the random components of distortions within a designed allowable range. According to this technique, an optical correction plate is inserted in a projection optical path. More specifically, an image distortion characteristic of a projection optical system assembled is actually measured, and the optical correction plate is polished to partially deflect the chief ray passing through respective points of the projection field of view so that the actually measured image distortion characteristic is minimized at respective points of the projection field of view.
While the aforementioned document (No. 8-203805) discloses a correction method related to a stepper employing an optical correction plate, Japanese Patent Application Laid-Open No. 11-045842 discloses a correction method employing an optical correction plate in a scanning projection exposure apparatus. The document (No. 11-045842) is directed to the fact that, when a pattern on a mask is exposed to a photosensitive substrate by a scanning projection exposure apparatus, an image distortion characteristic that is static with respect to the scanning direction throughout the projection area is averaged and becomes dynamic. Among the dynamic image distortion characteristic, at least random components are corrected by providing an image distortion correction plate in a projection optical path, i.e., a transparent plane parallel plate (optical correction plate) whose surface is locally polished.
In semiconductor device manufacturing processes, there are cases in which a pellicle is provided on one of or both surfaces of a reticle in order to prevent the transfer of a pattern on a reticle from being performed with attachment of foreign substances, such as dust, to the reticle or prevent such foreign substances from being transferred to a wafer.
The pellicle is configured with pellicle film (light transmissive dustproof film) which is held away from the reticle surface by a predetermined distance by a holding frame, e.g., aluminum. Therefore, when a wiring pattern formed on a pattern surface of the reticle is to be transferred to a wafer, even if a foreign substance is attached to the pellicle film, the foreign substance, e.g., dust, will not be transferred to the wafer because the reticle pattern surface and the pellicle film surface have different image-formation focusing distances. Therefore, by virtue of covering the reticle pattern surface with a pellicle, it is possible to prevent invasion of foreign substances, and improve a yield ratio in the semiconductor device production.
It is required for the pellicle film, employed as a pellicle, to have a sufficient film strength, light resistance, and light transmissibility in the wavelength of an exposure light source. As a pellicle film material for the conventional light, such as γ ray (436 nm), i ray (365 nm) or the like, cellulosic materials, e.g., cellulose nitrate, cellulose propionate or the like, are mainly employed.
Meanwhile, in the semiconductor device manufacturing processes, the trend is proceeding to shorten the wavelength of an exposure light source in order to improve the degree of integration accompanied by downsizing of micropatterns. To be more specific, currently a manufacturing process using a KrF excimer laser (wavelength of 248 nm) as an exposure light source is achieved. A manufacturing process using an ArF excimer laser (wavelength of 193 nm) as an actual exposure light source is in a process of being achieved as well. Also, utilizing ultraviolet rays having a shorter wavelength is under study. In particular, an F2 laser (wavelength of 157 nm) is regarded as most promising.
As a pellicle film material which is durable to such light sources having a short wavelength, a fluororesin having a relatively low absorptivity in a short-wavelength ultraviolet area is known. As an example, CYTOP (trademark of Asahi Glass Co., Ltd.) or Teflon AF (trademark of DuPont) may be given.
These fluororesins have excellent light transmissibility and light resistance when a KrF excimer laser or an ArF excimer laser is used as an exposure light source. However, when an F2 laser is used as an exposure light source, the resins cannot achieve sufficient light transmissibility. In addition, these resins are easily deteriorated by laser irradiation, and cannot be actually used as a pellicle. In view of this, Japanese Patent Application Laid-Open No. 2001-305719 discloses a pellicle having a high light transmissibility in the wavelength of 157 nm, and having excellent light resistance and film strength, which can be used when an F2 laser is used as an exposure light source. This pellicle is characterized by employing as a pellicle film a synthetic quartz glass plate, having the hydroxyl content of 20 ppm or less. According to this document (No. 2001-305719), keeping the hydroxyl content to 20 ppm or less, preferably 10 ppm or less, can control variations of transmissivity to 1% or less.
Japanese Patent Application Laid-Open No. 10-228099 proposes another example. The document (No. 10-228099) discloses a pellicle and a pellicle case, having excellent light resistance, for ultraviolet rays having a wavelength of 200 nm or less. The pellicle and pellicle case are controlled so that irradiation of ultraviolet rays do not cause the components produced from an adhesion layer, which is generated for fixing the pellicle film to the pellicle holding frame, to become an aromatic compound, ketone, or a nitrogen compound. As a film material, the document proposes to use polytetrafluoro-ethylene (PTFE), a fluorocarbon resin such as a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), a silicon polymer such as dimethyl polysiloxane, a fluorosilicon polymer and the like. Furthermore, commercially available fluorocarbon resins, such as CYTOP (trademark), Teflon AF (trademark) or the like, can be used.
Meanwhile, Japanese Patent Application Laid-Open No. 10-27738 discloses a calibration method of a scanning projection exposure apparatus. According to the document (No. 10-27738), a mask reference plate is arranged on a mask stage, a mark on the mask reference plate and a mark on a wafer reference plate on a wafer stage are detected by a microscope, and relative positions are measured. This configuration enables positioning of a mask and a wafer without a reference mask.
In a projection optical system of an exposure apparatus, it is desirable that both the reticle side (object side) and wafer side (image side) have a telecentric system. However, since it is difficult to completely remove a spherical aberration from the pupil of the projection optical system, there is a problem in that an angle between the wafer and the chief ray of an illumination light flux used at the time of examining the wafer mark is deviated from a vertical line. Assuming that an alignment mark on a wafer surface is to be examined through the projection optical system, if the alignment mark is examined only at a particular position (image height) on the optical axis of the projection optical system, the optical system is set so that the chief ray of the illumination light becomes vertical to the wafer surface at the image height. However, there are cases that the measuring image height must be moved because of the mark arrangement on the reticle or reference plate at the time of performing base line measurement for off-axis alignment, or that the examining image height must be changed for performing TTL on-axis alignment. In such case, if a spherical aberration remains on the pupil of the projection optical system, the angle between the wafer and the chief ray of the illumination light flux becomes deviated from a vertical line. When an incident angle of the examination light onto the wafer inclines, if the light is defocused to the + side or the − side, the alignment mark position measurement values will be deviated in correspondence with the inclination of the incident angle.
To solve this problem, the Applicant of the present invention proposes a method disclosed in Japanese Patent Application Laid-Open No. 8-262747. The document (No. 8-262747) is characterized by having a correction optical system for adjusting an optical path of a chief ray of examination light.
The conventional technique of adjusting an aberration characteristic by rotating a lens device serving as a part of the projection optical system or adding eccentricity or gradient with respect to the optical axis does not always ensure an excellent aberration characteristic (image distortion characteristic). Furthermore, in such adjusting technique, it is difficult to ensure stable precision. Moreover, the adjusting operation accompanies cumbersome trial-and-error elements which are problematic. The foremost problem of this technique is that although the technique enables uniform adjustment and correction on the overall image distortion characteristic in the effective projection area of the projection optical system, it cannot partially adjust or correct a local image distortion characteristic in the effective projection area.
In view of this, with respect to a scanning projection exposure apparatus, an optical correction plate is produced by the method disclosed in Japanese Patent Application Laid-Open No. 11-045842 and inserted in a projection optical path. It is predicted that inserting the optical correction plate enables easy correction of the local image distortion characteristic in the effective projection area.
In exposure light having a short wavelength, particularly in a laser having a wavelength of 200 nm or less, e.g., an F2 laser (wavelength of 157 nm), it has been found that the conventional pellicle film materials cannot be used. However, by utilizing a synthetic quartz glass plate as disclosed in Japanese Patent Application Laid-Open No. 2001-305719, it is possible to provide a pellicle having a high light transmissibility, excellent light resistance and film strength even if an F2 laser is used as an exposure light source. Moreover, Japanese Patent Application Laid-Open No. 10-228099 discloses a pellicle film material and a pellicle case, having excellent light resistance, for ultraviolet rays having a wavelength of 200 nm or less.
As described above, in a case where exposure light adopts an ArF laser or an F2 laser having a short wavelength, the use of a synthetic quartz glass material as a pellicle material is expected to increase, in addition to the use of conventional film materials. As disclosed in Japanese Patent Application Laid-Open No. 2001-305719, the thickness of a synthetic quartz glass material is larger than the conventional film material. Therefore, an exposure apparatus employing an ArF laser or an F2 laser as a light source must be compatible to both a reticle using a conventional film material as a pellicle and a reticle using a synthetic quartz glass material as a pellicle.
However, the film material and glass plate material have different thicknesses. Therefore, if an exposure apparatus which is supposed to employ a film material performs exposure with a reticle using a glass material as a pellicle, a pattern image of the reticle is transferred to a wafer with a focus deviation that corresponds to the thickness of the glass material (strictly speaking, corresponds to an increase in an optical path length caused by a refractive index of the glass material).
Furthermore, in employing a glass plate material, if the thickness of the glass plate material is not uniform on the entire surface, but is partially different or different among reticles (pellicles), similar consequences will result.
Meanwhile, in a scanning projection exposure apparatus, as disclosed in Japanese Patent Application Laid-Open No. 10-27738, a reticle (mask) reference plate is arranged on a reticle stage, a mark on the reticle reference plate and a mark on a wafer reference plate on a wafer stage are detected by a microscope, and relative positions are measured. Therefore, positioning of a mask and a wafer can be realized without a reference mask. However, if a correction plate proposed by Japanese Patent Application Laid-Open No. 11-045842 is employed, the existence or absence of the correction plate at the time of examining the reticle (mask) reference plate will cause a difference in the focus, ultimately deteriorating the measuring precision. Furthermore, changes in the pellicle thickness will also cause a similar problem.
Furthermore, even if a spherical aberration remains on the pupil of the projection optical system, it is possible to solve the problem by the method disclosed in Japanese Patent Application Laid-Open No. 8-262747 which proposes a correction optical system for adjusting an optical path of a chief ray of examination light. However, along with the improved precision, higher adjustment precision is required. As a result, the size of the adjustment mechanism is enlarged.
In view of the above problems, it is expected to develop an optical device which can correct influences of the surface shape of a reticle pattern, which is a distortion factor, and a random distortion that remains in the projection optical system, as well as a pellicle which is not easily deteriorated by a light source using lasers having a short wavelength, such as an ArF laser or an F2 laser.